As others have mentioned, all the multiplayer tricks from Crackdown 1 are still usable and C2 adds the "hang-from-a-chopper-using-mag-grenades" method.
However, all of the stunt rings are fairly do-able normally as a single player, and IMO easier than in Crackdown 1. All but about 5-6 of them require you to use the Agency SUV, and use its "Bunny Hop" capability. Most of those involve getting up a good running start towards a game-provided ramp somewhere (there's ALWAYS some sort of ramp provided) and launching the hop just before you get airborne. After you've done it a few times, it becomes second nature.
The most difficult ones are in Green Bay, on the beach and at the broken ferris wheel--in both cases you need to start WAY back to get enough speed up to clear the last ring without falling below it. That makes it tricky to line up the jump, especially on the sand where the rolling terrain tends to bounce you around a little. For that one, press and hold the "B" button just before you hit the last sand dune before the ramp to engage the "Super-Grip" over the last hill to prevent you from being bounced off-course.
Another difficult one is in Green Bay (on the "other", northern island) just in front of the stadium because the rings are positioned at an angle that prevents you from getting a good running start. But there's a Ramp Truck right there that will help (although it can be done using the rocks that have fallen on the road and without the ramp truck). I found that I was able to hit the first two rings using the rocks as a ramp, then use the SUV's "Super Grip" ability (hold B without releasing) to drive up the wall as if to climb onto the nearby overpass. Part way up the wall I touched the third ring and completed the stunt.
There are two that are launched from the top of the parking garage in Green Bay. They are fairly straight-forward, but tricky to line up AND get enough speed, because both require you to make a quick turn at the last moment before launching.
The rest I found fairly easy. If it's a single ring, look for the closest, highest place to jump at it from. You might need to carry or throw the SUV up onto the top of a building or hill to get there. If it's a multi-ring set, then look for a suitable (high-altitude) place in the opposite direction of the first ring to launch from. And remember that most of these allow you to miss the last ring on your main jump, but immediately turn around and do a Bunny Hop to catch it within the time limit.
Finally, keep in mind that there's one set of rings in Unity Heights--on a broken bridge--that you might want to hold off getting until after you've gotten the "Party Bus" achievement.